The present invention relates to floor cranes and more particularly to mobile floor cranes used in the automotive aftermarket.
Conventional mobile floor cranes such as shown in U.S. Pat. Nos. 3,931,956 and 4,669,703 typically have a pair of legs and a cross piece forming a bridge connecting the two legs and supporting the central upright stanchion of the crane. The legs and the cross piece typically are formed of tubular steel for increased strength. However, the strength of the cross piece and the strength of its connection to the two legs limit the amount of weight that can be lifted by the crane. The lateral distance between the stanchion and the legs provides a bending moment that can apply a twisting force to the legs and cause the crane to fail. Attempts to stabilize the legs against twisting have included the provision of a caster under each leg in the vicinity of the crosspiece as shown in U.S. Pat. No. 5,076,448. However, this then shifts the load from being carried by the rear wheels to being carried by the wheels beneath the legs by the crosspiece, and this shift has the undesirable effect of reducing the overall footprint of the load-carrying components of the crane. In the end, one type of instability is traded for another type of instability.
Moreover, construction of these so-called bridge-type cranes wherein the cross piece forms a bridge between the two legs, involves multiple manufacturing operations like metal cutting, hole-drilling, positioning, welding, and bolting. These manufacturing operations, and particularly the welding operations, add significantly to the overall cost of the crane. For example, the cross piece must be sized and cut, and in some embodiments the cross piece must be welded to the base and/or the legs. A crane design that could eliminate the cross piece might be produced less expensively than a comparable bridge-type crane due to the elimination of fabrication materials, fabrication time, and fabrication operations involving the cross piece.
Precise positioning of the parts relative to one another before they are welded also plays a significant role in the cost of manufacture of these conventional cranes. If the upright stanchion is located off-center relative to the two legs, then the load carried by the stanchion will not be evenly distributed between both legs of the crane. The off-center stanchion may wiggle or tend to tilt in use. Thus, siting of the stanchion atop the cross piece must be done with care, or the stanchion will be off center and the crane will need to be rejected. Rejects lead to waste that increases the cost of production. Moreover, the cumulative tolerances for the parts involved in positioning the stanchion can result in errors that cause a misalignment that might not be detectable by the eye of the user who assembles the crane.